The present invention relates to operational amplifiers and, more particularly, to a precision trimming circuit that provides input offset voltage compensation.
An operational amplifier (op amp), which is a basic building block in analog integrated circuits, amplifies the difference between two different potentials. The basic op amp includes a dc amplifier with a differential input and a single-ended output. Ideally, the op amp has a zero output voltage for zero input. However, because of the inherent lack of precision in matching the op amp""s two differential input transistors, the op amp may have some output voltage for zero input. The voltage applied to the differential input that will make the output voltage zero is called xe2x80x9cinput offset voltage.xe2x80x9d
The magnitude of the input offset voltage can severely limit the applications in which the op amp can be used. The input offset voltage can be canceled by equal and opposite compensating signals. The difficulty with input offset voltage is that it can change with temperature. This change in input offset voltage with temperature is called xe2x80x9cthermal drift.xe2x80x9d Thus, in order to maintain the performance of the op amp within specified criteria, the offset compensation mechanism should be correlated to thermal drift. This problem is especially severe in complementary metal-oxide semiconductor (CMOS) integrated circuits which have well-known consumption and speed benefits. Thus, what is required is to maintain the input offset voltage over the operating temperature range after the device has been trimmed.
The present invention provides compensation for input offset voltage by balancing the operational amplifier over the operating temperature range after the device has been initially trimmed. The present invention employs a low input offset voltage which remains low over the operating temperature range without a separate temperature compensation circuit. The present invention has a separate trim device for each current path of the circuit to maintain symmetry. Thus, the current paths of the differential circuit have the same leakage current upon temperature excursions. Ideally, the leakage current will occur in both current paths of the differential circuit and maintain circuit balance.